Factor VIII is a large (˜300 kDa) glycoprotein that functions as an integral component of the intrinsic pathway of blood coagulation. It contains a series of domains designated A1-A2-B-ap-A3-C1-C2. The B domain of factor VIII has no known function and can be deleted without loss of coagulant activity. Mutations in the factor VIII gene that result in decreased or defective factor VIII protein give rise to the genetic disease, hemophilia A, which is characterized by recurrent bleeding episodes. Treatment of hemophilia A requires intravenous infusion of either plasma-derived or recombinant factor VIII.
Since the introduction of recombinant factor VIII for the treatment of hemophilia A, supply has struggled to keep up with demand because factor VIII is expressed and recovered at low levels in the heterologous mammalian cell culture systems used for commercial manufacture (Garber et al. (2000) Nature Biotechnology 18: 1133). Additionally, factor VIII levels during hemophilia A gene therapy trials indicate that expression levels will be a limiting feature (Roth, et al. (2001) N. Engl. J. Med. 344:1735-1742). The importance of this problem has resulted in significant research efforts to overcome the low factor VIII expression barrier. Several factors that limit expression have been identified, including low mRNA levels (Lynch et al. (1993) Hum. Gene Ther. 4:259-272; Hoeben et al. (1995) Blood 85:2447-2454; Koeberl et al. (1995) Hum. Gene Ther. 6:469-479), interaction with protein chaperones and inefficient secretion (Pipe et al. (1998) J. Biol. Chem. 273:8537-8544; Tagliavacca et al. (2000) Biochemistry 39:1973-1981; Kaufman et al. (1997) Blood Coagul Fibrinolysis 8 Suppl 2:S3-14) and rapid decay in the absence of von Willebrand factor (Kaufman et al. (1988) J. Biol. Chem. 263:6352-6362 and Kaufman et al. (1989) Mol. Cell. Biol. 9:1233-1242). Deletion of the B-domain has been shown to increase factor VIII protein production in heterologous systems (Toole et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83:5939-5942). A B-domain deleted form of human factor VIII (Lind et al. (1995) Eur. J. Biochem. 232:19-27) has been approved for clinical use.
Despite these insights into factor VIII regulation, expression continues to be significantly lower than other recombinant proteins in the heterologous systems used in commercial manufacture (Kaufman et al. (1997) Blood Coagul. Fibrinolysis 8 Suppl 2:S3-14), as well as in ex-vivo (Roth, et al. (2001) N. Engl. J. Med. 344:1735-1742) and in vivo gene therapy applications (Chuah et al. (1995) Hum. Gene Ther. 6:1363-1377). Methods and compositions are needed for the increased expression of factor VIII.